The “Conversion to shorter data type” warning occurs in an expression where the lvalue (left-value) is of a shorter type than the rvalue (right-value). The warning is about this conversion of data to a shorter type.

Here are a few examples that might help you understand why it occurs in your code:

Example 1:

Example 2:

The buffTail expression is a defined as type uint8_t.

In the buffSize-1 expression, the constant 1 will be treated as an int type, and the resultant expression too will have type int.

In the expression buffTail &= (buffSize-1), as the lvalue is of shorter type (uint8_t) than the rvalue (int), the conversion to shorter data type warning will be triggered.

Example 3:

Constant values are treated as type int. The resultant of the expression 0xFF - cycleCount will be of int type. Since the lvalue (i.e., TMR0) is of shorter type (volatile unsigned char) than the rvalue (int), the conversion to shorter data type warning will be triggered.

Example 4:

The resultant value of the expression (BAUD9600»8) is of int type. Since BAUD9600 is a constant, the resultant value of the expression will also be int type. Assigning an int type to a char type (SPBRGL), will result in the conversion to shorter data type warning.

We recommend you review all compiler warnings before testing or utilizing your code.
However, if you want to suppress this warning, you can do so by using the --MSGDISABLE=752 option.

Under MPLAB X IDE Project Properties > XC8 global options > Additional options: <specify the option in the dialog box>
--MSGDISABLE=752